Mechanical drives are accustomed to transmit motion, torque and power from a driver shaft to powered shaft. The driver shaft, in majority of the cases, is part of prime mover (such as electric motor, hydraulic turbine, steam turbine, etc.); while, the driven shaft is a part of the machine device. There exist four basic mechanical drives, namely gear drive, belt drive, chain drive and rope drive. A gear drive is 1 engagement type rigid drive where motion and power are transmitted by means of successive engagement and disengagement of teeth of two mating gears. It really is inherently free from slip and this it provides constant velocity ratio (positive drive). It can be used for light duty applications (such as for example toys, watches, etc.) aswell as for durable applications (such as for example gear container of machinery, marine drive, etc.).

Driver and driven shafts might have 3 mutual orientations, namely (we) parallel shafts, (ii) intersecting shafts and (iii) nonparallel nonintersecting shafts. There exist four simple types of gears and the right gear ought to be selected based on the mutual orientation of the driver and driven shafts. Spur equipment and helical gear are applicable for parallel shafts. Bevel gear can be applied for two intersecting shafts, which may not necessarily be perpendicular. Worm equipment arrangement is used for the third category (nonparallel non-intersecting shafts). Unlike spur gears that have straight the teeth parallel to the apparatus axis, helical gears have tooth in helical form that are cut on the pitch cylinder. Although helical gears are commonly utilized for parallel shafts like spur gears, it may also be used for perpendicular but non-intersecting shafts.

Accordingly there are two types of helical gears-parallel and crossed. Parallel helical gears, the common one, is utilized to for power transmission between parallel shafts. Two mating parallel helical gears should have same module, same pressure angle but opposite hand of helix. They provide vibration-free and quiet operation and will transmit heavy load. On the other hand, crossed helical gears are utilized for non-intersecting but perpendicular shafts. Two mating crossed helical gears (also known as screw gears) should have same module, same pressure angle and either same or opposite hand of helix. This type of gear has program similar to worm equipment; however, worm gear is preferred for steep speed reduction (1:15 to at least one 1:100), whereas crossed helical gears cannot offer quickness reduction beyond 1:2. Various variations between parallel helical gear and crossed helical gear are given below in desk format.